The present invention relates to an apparatus for limiting an electrical current in an electrical line. The apparatus comprises a PTC resistor to effect said limiting function.
A PTC resistor as such is state of the art and well known to have a transition between a low resistance status and a high resistance status that is temperature dependent and thus can be triggered by the amount of current carried by the PTC resistor.
Further, the apparatus according to the invention comprises a monitor circuit for monitoring the status of the PTC resistor.
Monitoring the status of PTC resistors is known from DE 43 40 632 A1. Therein the voltage across a PTC resistor being connected in series with a load break switch is used to trigger the opening of the load break switch. The PTC resistor is intended to improve the current limiting capability of the load break switch which finally interrupts the current.
Prior art U.S. Pat. No. 5,530,613 discloses an apparatus for limiting an electrical current in an electrical line. According to this document, triggering of PTC resistors is monitored. In case of triggering, a switch shall be opened that is exemplified by a contactor. This contactor is used for short-circuit current interruption. Another embodiment shows a combination of a contactor and a circuit interrupter switch, however, without a monitor circuit for monitoring the PTC resistors. The additional switch is provided in order to have independent means for disconnecting.
Further, DE 42 13 443 and DE 41 18 346 are mentioned which show monitoring of conventional fuses.
The technical problem underlying the present invention is to provide an apparatus for limiting an electrical current with improved efficiency.
According to the invention, this technical problem is solved by an apparatus for limiting an electrical current in an electrical line comprising a PTC resistor having, alternatively, a high and a low resistance status, in said line and a monitor circuit for, in case of a change from said low resistance status to said high resistance status, detecting the status and producing a signal if said high resistance status is present, a switch in said line and in series connection with said PTC-resistor for interrupting an electrical current in said line limited by said PTC resistor, responsive to said signal indicating said high resistance status, characterized in that said monitor circuit is adapted to detect said resistance status for a defined time period after said change and to produce said signal only if that high resistance status is present for said time period and that said apparatus is adapted to open said switch in case of high overcurrents and further comprises a contactor to be opened in case of small overcurrents.
The basic idea of the invention is that a change of the PTC resistor from its low resistance status to its high resistance status does not necessarily indicate a situation, where the monitor circuit should issue a signal. The signal can be used for signalizing purposes, e. g. to inform a control and supervision centre or to drive a lamp or display, additionaly to being used to activate another circuit, e. g. to open a switch. The invention is based on the experience that some of the situations leading to a status change of the PTC resistor can be handled by means of the PTC resistor only. E. g. short overcurrents can eventually trigger the PTC resistance change without really posing a problem that goes beyond the current limiting capabilities of the PTC resistor.
In contrast to the cited prior art wherein PTC resistors are monitored in order to open a switch in every occurring case of a PTC resistance change, the invention is based on the statement that in some of these situations the opening of the switch is not necessary, actually. In other words: The cited prior art regards the PTC resistor to be an additional element for improving the current limiting capability of a switch wherein the interrupting action of the switch shall only be improved (but not replaced). The invention, to the contrary, regards the PTC resistor to be a functional device for itself. That means that the invention does not aim at triggering the switch in each case of a PTC resistance change.
According to the invention, the measure to discriminate between those cases of a PTC resistance change in which a signal shall be issued by the monitor circuit and the remaining cases in which the monitor circuit shall not respond on such a change, is given by a time period of continued detection of the status of the PTC resistor after the resistance change of the PTC resistor. This time period shall be a defined one which preferably means a predeterminedxe2x80x94but with this predetermination fixedxe2x80x94time period, but can also mean a time period varied in some defined way. Within this time period, preferentially, the status of the PTC resistor is monitored by means of detecting the voltage across the PTC resistor. The advantage of these solutions is that they enable a very rapid monitoring of the PTC resistor status compared to other alternatives as temperature detection, detection of mechanical changes such as increasing length or others. Namely, in case of an overcurrent through the PTC resistor lasting only for a short time, the voltage across the PTC resistor will drop as soon as the overcurrent situation is finished. The material changes within the PTC resistor could, however, last for longer time, i. e. the cooling of the PTC material could possibly need more time than the actual overcurrent situation has taken. Therefore, if the time delay according to the invention shall be shortened, it might happen that only by detecting the voltage during that time period it can really be decided, whether at the end of that time period the overcurrent situation is still present or not.
As stated above, the invention relates to a combination of the PTC resistor with the monitor circuit and a switch in line with the PTC resistor, controlled by the signal of said monitor circuit. Such a switch could be a load break switch or switch fuse. The large current interrupting capability of a real circuit breaker switch is not necessary in each application of the invention, mostly because the PTC resistor reduces the current to be switched after the PTC resistance change. However, the invention can also be worked with a circuit breaker. An advantage of this could be that one does not depend on the current limiting function of the PTC resistor which could fail, principally.
Additionally to a load break switch, a circuit breaker or another switch, a contactor is used in series with the switch and the PTC resistor for two main reasons: Contactors have large duty life values, i. e. they are adapted to large numbers of switching operations. Therefore, they are suitable for an (intentional) operational on/off switching of a load which usually occurs much more often than overload situations and can be operated from a supervision center. Further, such a contactor shall be driven by a thermal relay responding on small overcurrents in the same line which are below the threshold value of the PTC resistor but, if continued, too high to be tolerated.
According to a preferred embodiment of the invention, the resistance status of the PTC-resistor is not only used to produce a signal for opening the switch, but moreover to produce a signal for opening a contactor only in case of only small overcurrents.
A suitable choice for the PTC resistor is a PTC-polymer resistor as known in the art.
Furthermore a PTC polymer resistor has been proposed with two filling powders, one of a conductive material and the other one of a phase transition material. The phase transition material absorbs a phase transition energy at a phase transition point which is at a lower critical temperature than the critical temperature of the PTC resistance change. Thereby, an adaption of the PTC response characteristic to the start-up characteristics of an electrical motor can be achieved.
Still further, another preferred aspect of the invention relates to a circuit design to detect the voltage cross a three-phase PTC resistor by using two star points, each combining three taps of in total six taps on both sides of the three-phase PTC resistor. This aspect of the invention is also described hereunder.